The present invention relates to a loop antenna device and more particularly to a loop antenna device preferably used for a reader/writer device of an electromagnetic induction type non-contact data carrier system in which data transmission and reception are executed in a non-contact state between a plurality of data carriers and the loop antenna device.
In general, a data carrier system includes a reader/writer device and a data carrier. An antenna device of the reader/writer device supplies a modulated alternating magnetic field to a service area to transmit data or a command to the data carrier.
The data carrier receives the modulated alternating magnetic field supplied to the service area using an incorporated coil to generate electromotive voltage that is rectified and used as an operating power source of the data carrier. The data carrier also executes prescribed operations, such as demodulation of the modulated alternating magnetic field for retrieving the data or the command sent from the reader/writer device and transmission of data to the reader/writer in accordance with the command.
Such a data carrier system is required to restrict the intensity of the alternating magnetic field supplied to the prescribed service area from the reader/writer within a prescribed range and to make a magnetic filed generated outside the service area as weak as possible.
The reason why an alternating magnetic field sufficiently strong but falling within the prescribed range needs to be supplied to the service area would be clear. The reason why the alternating magnetic field that reaches outside the service area is desirably as weak as possible is to eliminate interference with or adverse effect to another device. For example, when a plurality of the above-described reader/writer devices are arranged and installed adjacent to one another in, for instance, an automatic ticket-examination system of a station, it is necessary to eliminate interference with or an adverse effect to each other in operation. As this kind of the antenna device as a related art, for instance, a device disclosed in Patent Document 1 is known.
The antenna device of the related art is constructed by a plurality of loop antenna cells which are combined together under below-described conditions in order to satisfy the above-described requirements.
      Formula    ⁢                  ⁢    1                                                          ∑                              i                =                0                                            i                =                n                                      ⁢                                                  ⁢            NiIiSi                    =          0                                      (                      1            ⁢                                )                    
Here, Ni designates the number of windings of each loop antenna cell. Ii indicates an electric current supplied to each loop antenna cell and is designated by attaching plus and minus depending on its supplied direction. Si indicates an area of each loop antenna cell. “NiIiSi” indicates a magnetic moment generated by an i-th loop antenna cell.
The above-described loop antenna device of the related art has employed such a phenomenon that the alternating magnetic field reaching a part remote from the size of each loop antenna cell can be weakened. From a measurement test in which the intensity of the alternating magnetic field generated from such a loop antenna device is measured, it was confirmed that the intensity of the alternating magnetic field is drastically lowered as the alternating magnetic field is remote from the size of the loop.
Patent Document 1: JP-A-10-209737
However, the measurement test also showed that the above-noted advantage can be obtained stably only when the loop antenna device alone is disposed in a measurement environment. That is, even if the loop antenna device is designed in such a way that the intensity of the alternating magnetic field is abruptly lowered when the alternating field is remote from the loop size as described above, the influence of an environment around the loop antenna device causes an imbalance between the loop antenna cells, resulting in a strong magnetic field undesirably reaching a position remote from the loop size.
Specifically, when metal or the like is a symmetrically disposed in the vicinity of the loop antenna device, induced current may flow through the metal or the like to generate a magnetic field which deteriorates the intended balance between the loop antenna cells in the loop antenna device . Thus, when the loop antenna device is assembled into the reader/writer device, the intended balance between the loop antenna cells may be undesirably deteriorated.
Further, even if the loop antenna device is assembled into the reader/writer device with good balance, it would be practically impossible to maintain or obtain the balance, taking into account the environment around the loop antenna device, when the reader/writer device is installed.
Further, when a tag is placed on a specific one of the antenna cells, induced current flowing through a coil of the tag also causes a problem in which the balance between the loop antenna cells may be deteriorated similarly to the above-described case. In short, the solution in which the plurality of loop antenna cells are combined together to adjust the sum total of the magnetic moments to be “0” still suffers from a problem in which the intended balance between the loop antenna cells is readily deteriorated in practical use.
With the above-described problems taken into consideration, it is an object of the present invention to provide a loop antenna device having a plurality of small antenna cells combined together, which can compensate for imbalance caused between loop antenna cells by the change of an operating environment and which can constantly reduce the sum total of magnetic moments generated from the loop antenna cells to be a minimum, preferably “0”.